Summary of Significant Accounting Policies - Additional Information (Detail) (USD $) In Millions, unless otherwise specified					12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2013 Alterra Common Stock [Member]	Dec. 31, 2013 Alterra Warrants [Member]	Dec. 31, 2013 Markel Corporation [Member]
Conversion of Stock [Line Items]
Holdings of common stock and warrants, cost basis					$ 177	$ 63
Transferred, common stock value							226
Cash	$ 52	$ 50	$ 58	$ 70			$ 98